When creating a custom CellHighlightTemplate for a ColumnDefinition which MemberBinding is set to a custom property or a property that is not the Start, End or Title properties of the GanttTask you should set the binding in the DataTemplate to the built-in FormattedValue property as the above code snippet shows. This is an optimized string value that comes from the built-in virtualization of the control in order to achieve maximum performance.

You can download a runnable project of the previous example from our online SDK repository here, the example is listed as GanttView / CustomCellTemplates.